The invention is in the field of control systems for maintaining air temperature comfort in various structures. More specifically, it is an air temperature control system for operating supply fans for reductions in electrical energy consumption.
Since the energy crisis of the early 1970's, commercial structures, i.e., office buildings, retail stores, etc., which have modified their older air handling equipment or installed new equipment, have incorporated systems which are controlled to "load shed" at a predetermined power demand. In a commercial structure having, for example, six air handling units controlled by a load-shedding system to maintain store comfort, up to two of the units may be shed and cycled at any one time to regulate store comfort. These load shedding systems have proved not to be ideal for efficiently maintaining commercial comfort in these types of structures. Since the greatest power demand occurs during the summer, the load shedding system is designed for maximum summer conditions. Thus, during most of the year the flow of conditioned air through the air handling units may, for example, be two and a half times greater than necessary. This increased capacity requires a greater initial equipment investment and increased maintenance costs over the operating life of the system. Further, operation of the load shed system results in heavy belt and motor starter bearing wear caused by the continuous on/off cycling of each of the motors of the air handling units. Moreover, the air flow throughout the system may not be evenly distributed as the individual units load shed. Another not insubstantial problem associated with the load shedding arrangement is air duct noise occurring every time a unit comes on line, which may cause occupants of the structure annoyance. In addition to the load shedding system, further examples of air temperature control systems are described in Sparrow, U.S. Pat. No. 2,805,026; Elwart U.S. Pat. No. 3,454,078; and Denny U.S. Pat. No. 4,007,605. Sparrow relates to a temperature control system for a plurality of znes for a hot air furnace system to be used during the winter to individually control the temperature in each zone. A series of dampers in each zone controls the air flow provided by a continuously operating blower. Elwart discloses a heating and cooling system. When the heating system is used, a variation in resistance varies the intensity of the current in the blower fan circuit thus varying the speed of the blower fan. The Elwart blower in the heating mode is a variable speed device and is operated in conjunction with the furnace, i.e., the fan speed increases as the furnace temperature increases. When the air conditioner mode is in operation, the blower fan is not variable and operates at top speed only. Denny discloses a system for regulating air flow over the outside of condenser units of an air conditioning system. Denny is limited to refrigeration systems and is designed to optimize cooling.
In view of the prior art, there is a need for an air temperature control system that will operate at greater energy savings than present systems of the cycling and shedding type and other types described hereinabove. Further, there is a need for a system where the air flow is evenly distributed throughout the zones of the structure and annoying air duct noise is eliminated. Moreover, there is a need for a system that is equally effective throughout the year, i.e., heating, ventilating or air conditioning.
This invention is directed to a two-speed, two-winding motor control in conjunction with a two-speed, two-winding starter for obtaining reductions in electrical energy consumption. Control is based on fan speed reduction and not demand reduction. The general purpose of this control is to operate temperature conditioning supply fans at low speed or automatic thermostatic two-speed control (high or low), depending on an operator's decision through a selector switch. In either mode of operation, the fan system will operate at a greater energy-savings than present systems including fan cycling and shedding type control.
The subject invention relies on a motor speed reduction from the maximum rated speed of the air handling unit supply fans to supply air through a zone continuously without requiring cycling of the fan. The amount of air delivered by the unit is proportional to the speed of the fan. According to basic fan law, the reduction in horsepower, due to a reduction in fan speed, is proportional to the third power of the fan speed variation. In mathematical terms, the relationship may be expressed as follows: EQU [HP.sub.1 /HP.sub.o ]=[Q.sub.1 /Q.sub.o ].sup.3 (Equation 1)
Where HP.sub.o is the initial horsepower, HP.sub.1 is the reduced horsepower, Q.sub.o is the corresponding initial fan speed and Q.sub.1 is the corresponding new reduced fan speed. For example, if the supply fan is operated at 2/3 of the rated maximum speed of the fan, the corresponding reduced horsepower required to operate the fan, according to Equation 1, is: EQU [HP.sub.1 /HP.sub.o ]=[2/3].sup.3 =8/27
Thus, the horsepower requirement to operate the fan at two-thirds speed is equal to approximately 30% of that required at full speed. Considering six similar air handling units in a structure, the horsepower savings described in Equation 1 would be proportionately increased. None of the prior art disclosures known by applicant are concerned with a reduction in energy consumption. Large savings in electric power consumption, as compared to the devices in the prior art, yet still allowing for optimum performance during peak season, i.e., summer, is accomplished by the subject invention.
In a specific embodiment of the invention, each zone of the structure includes temperature sensing bulbs for determining the ambient temperature in the zone and a thermostat for selecting a desired zone temperature. A plurality of damper motors controlling corresponding outside air dampers supply air into the zone for ventilation purposes, and whenever possible, to minimize the use of mechanical air cooling apparatus and consequently reducing energy consumption, outside air is brought into the zone in an attempt to cool the zone to the desired temperature. A heating coil, supplied by a hot water pump, during the heating season and a cooling coil, supplied by chilled water, during the cooling season temperature conditions the supplied air. A pneumatic control system which is responsive to the temperature selecting and temperature sensing means selectively actuates the heated, cooled or ventilated air supplied in the zone. A two-speed supply fan circulates the supplied air received through the outside air dampers and the conditioned air from the heating and cooling coils to the zone. The supply fan is controlled by the pneumatic control means to operate in a first state only at low speed, i.e., at two-thirds the rated maximum speed of the motor, to reduce energy consumption, or in a second state at automatic thermostat two-speed control (high or low). The fan in this second state operates at low speed except when maximum cooling is required and then automatically switches back to low speed operation from high speed when the ambient temperature in the zone approaches the desired temperature.
A starter having two windings is coupled to a set of corresponding motor windings for activating the supply fan. The two motor windings correspond to the high and low speeds of the fan. The use of the two-winding starter in conjunction with the two-speed two-winding motor control for the fan provides for an efficient transfer of energy and reduced energy consumption.